Anatomy of a car crash
Here's what happens, moment by moment, during most metal-to-metal mishaps.
Every car crash is uniquely destructive — from low-speed rear-end collisions, the most common type of mishap, to catastrophic high-speed head-ons. And despite the auto industry's attempts to prevent them, particularly through the widespread rollout of active safety systems, they still happen: 5.4 million auto collisions were reported in the United States in 2010. Consequently, automakers must prepare for the worst, studding vehicles with sensors inside and out to detect impending danger, and applying techniques both old and new to isolate vehicle occupants from the effects of an unexpected impact. Here is how some of those safety features react during a collision and at what moment.
5 seconds to impact: Forecasting disaster
Modern automobiles equipped with active safety systems are nervous Nellies, constantly scanning for threats with an array of long- and close-range sensors. At the furthest extreme is radar, which can detect a potential problem from nearly 500 feet away. Assuming that the questionable obstacle is stationary and you're traveling at 65 mph, this gives your car's onboard processors roughly five seconds to churn through likely scenarios and responses.
1 second to impact: Risk assessment
Within the final second before impact, short-range sensors — typically cameras, laser range finders or both — reclassify a vague, radar-detected risk as a specific threat. Depending on a vehicle's make and speed, the car might do anything from simply precharging its brakes to actively applying them. For instance, Mazda's Smart City Brake Support System, which debuted in this year's CX-5, tries to halt the SUV to avoid a crash, but only if its speed is between 3 and 19 mph.
— BAM! IMPACT! —
1 to 8 milliseconds after impact: Damage report
It takes roughly one millisecond for a vehicle's pressure sensors to register impact. With each additional millisecond, the severity of the crash becomes more apparent, as the kinetic energy flows through the car, tripping other internal sensors: acceleration, pressure, vibration. It's during this fraction of a second that commands are issued. Airbags are ordered to deploy, and seatbelt pretensioners are fired. Some cars even have features to minimize the effects of, say, whiplash; properly equipped Toyotas will have reconfigured their seats to reduce neck and back injury, shifting the headrests upward and forward.
8 to 15 milliseconds after impact: Airbags fire
Depending on where and how fast the vehicle has struck, or been struck, the internal airbags start inflating at the 8.5-millisecond mark. A front impact can mean a slight delay in deployment, to optimize the airbag's energy-absorbing properties. The exact timing depends on everything from the violence of the collision to the relative size of the occupants, as detected by seat-based weight sensors.
15 to 30 milliseconds after impact: Airbags inflate
By now, a high-speed crash has crushed a portion of the vehicle, dumping some of its force into corresponding crumple zones. Even if the cabin isn't forced inward, the passenger is at risk of bouncing against doors, steering wheel and dashboard. This is when most airbags fully deploy, exploding through their compartments to unfurl in a handful of milliseconds. Automakers have installed more airbags over the years, from front to side to knee-level, and General Motors recently introduced a front-center airbag on the 2013 Acadia, which will prevent occupants from clashing heads or otherwise moving around laterally during side collisions.
30 to 50 milliseconds after Impact: Airbags deflate
The goal of any passenger-based safety system is to cushion, to bleed off extreme and errant levels of kinetic energy. That's why seatbelts typically release some of their tension in a front-end crash, allowing the occupant to plow into the airbag at the precise moment that it starts deflating. Without that perfect transfer, the organs could slam forward inside the tightly held body, or the head could snap back against a rigid bag. Neither scenario is good for the body. To further muffle the blow in these scenarios, Ford has started offering inflatable seat belts, which deploy 40 milliseconds after impact.
50 to 70 milliseconds after impact: Controlled deformation
In most collisions this is the home stretch, when the forces involved have done their damage and internal movement tapers off. Modern vehicles are designed to yield in various ways, allowing the crash energy to essentially splash around the vehicle. "Specific deformation sends force around the safety cage," says Adam Kopstein, who manages safety compliance for Volvo in the U.S., referring to a typical front-end crash. "It travels up the A pillars, through the roof structure, then back down the vehicle around the occupant." Although the passenger might still be in motion, it's around this time that the doors automatically unlock.
70 to 300 milliseconds after kmpact: The crash's last gasp
If the collision hasn't caused a rollover, or secondary impacts — with other vehicles, for example — the crash is over. Occupants come to a full halt inside the cabin, and so do the structures designed to move with them. For many collisions, particularly side and rear impacts, this is often when the occupants first realize that a collision has occurred — when the bags have already deflated and the structure has finished warping itself around them.
While the wider auto industry continues to experiment with vehicle telemetry, remotely tracking the state and status of models on the road, GM's OnStar system provides the only factory-built post-crash safety feature. OnStar's Automatic Crash Response sends an alert to the service upon impact, allowing OnStar operators to pass along the location and, in general terms, nature of the crash to emergency responders. Since 2009, the service has added an Injury Severity Prediction feature, which uses sensor data to assess potential injuries, giving operators an estimated level of injury: high or normal.
Based out of the Boston area, Erik Sofge is frequent contributor to Popular Mechanics and Slate.com. He specializes in everything scientific and technical.